Electric resistance elements and the like



Patented May l4, 1946 ELECTRIC RESISTANCE ELEMENTS AND THE LIKE LeonardBessemer Pfeil, Birmingham, England, assignor to The InternationalNickel Company, Inc., New York, N. Y.', a corporation of Delaware NoDrawing. Application May 15, 1942, Serial No. 443,174. In Great BritainMay 27, 1941 Claims.

The present invention relates to heat-resisting alloys and articles madetherefrom and to a method of producing such alloys and articles, andmore particularly to heat-resisting alloys and articles having animproved combination of prop erties.

It is well known to incorporate in heat-resisting alloys, such, forexample, as those used for electrical resistance elements, smallquantities of one or more elements such as the rare earth metals,particularly cerium, the alkaline earth metals, particularly calcium,thorium, etc., in addition to those of which the alloy is basicallycomposed, for the purpose of improving the high temperature propertiesand in particular the service life of the alloys and articles madetherefrom. As an example, the heat-resisting properties, and inparticular the resistance to oxidation, of alloys consisting basicallyof 80% nickel and chromium are materially improved by the inclusion ofsmall quantities of calcium, cerium, calcium and cerium or calcium andthorium. Many other elements may be used and, for convenience, in

view of their capacity for increasing the life of the alloy under hightemperature conditions, they are referred to herein as long-lifeelements.

I have found that the long-life elements, when incorporated in thealloys to improve their life, may have an adverse effect on otherproperties of the alloys. In an increasing number of applications ofheat-resisting alloys, resistance to creep at high temperatures is anessential requirement, and long-life additions to the alloys mayseriously impair this resistance. Furthermore, the addition of thelong-life elements to the molten alloys may be a matter of difliculty,as many of the long-life elements are extremely reactive. Again, thepresence of long-life elements in the alloys may render them difficultto fabricate, and in some cases the alloys cannot be hot worked.

I have discovered that some of the elements will produce the desiredeffect of an increase in the service life of the basic alloys if theyare applied in the form of films to the surfaces of the alloys.

It is an object of the present invention to provide heat-resistingalloys and articles having an improved combination of properties.

It is another object of the present invention to provide heat-resistingalloys and articles made therefrom having high service life combinedwith improved resistance to creep.

It is a further object of the present invention to provide a novelmethod of improving the life are extremely reactive.

of heat-resisting alloys and articles made therefrom.

It is also an object of the present invention to provide a method ofimproving the life of heatresisting alloys and articles made therefromwithout seriously impairing the creep resistance of the alloy orarticle.

Other objects and advantages of the invention will become apparent tothose skilled in the art from the following description.

The elements which I have found may be applied as films are thorium,calcium, zirconium and the rare earth elements. According to thisinvention the alloys or articles that are made therefrom and by theirnature are necessarily subjected to high temperatures in service areprovided on their surfaces with films of one or more of these long-lifeelements in the elementary form or in the form of oxides or hydroxidesor in the form of salts that will decompose to oxides at hightemperatures.

The service life of the alloys or articles provided with such surfacefilms is not always so great as when the long-life element is includedin the alloy, but compensating advantages are often obtained. Forexample, the addition of the elements to the molten alloys may be amatter of difiiculty, as many of the long-life elements Again, thepresence of long-life elements in the alloys may render them diflicultto fabricate, and in some cases the alloys cannot be hot-worked. Thesedifiiculties are avoided altogether by means of the invention. In anincreasing number of applications of heatresisting alloys, resistance tocreep at high temperatures is an essential requirement, and longlifeadditions to the alloys may seriously impair this resistance. Surfaceapplication of the longlife elements has the great advantage of allowingof the production of articles and machine parts for high temperatureoperations having high resistance to. oxidation and without anyreduction in their normal resistance to creep.

It appears that the long-life elements produce the effect indicated bytheir influence on the' formation of scale on the surface of the alloy.Whether this is so or not, I find that what is necessary is to produce afine dispersion of the long-life element or elements on the surface.This need not be continuous, although the particles that compose itshould be uniformly distributed, and it must not be a thick coating suchas would either completely exclude oxygen from the surface or would tendto flake ofi under rapid changes of temperature. With such a thickcoating the desired efl'ect is not produced.

f the long-life elements defined above, the

best results are obtained with thorium and the rare earth elements. Thelatter may advantageously be used in the form of a so-called mischmetallcontaining cerium and other ele in, a solution of a salt of a long-lifeelement, or.

to deposit an oxide or hydroxide of the element anodically. Inthe formercase, the finished artivie should first be heated to give it a lightoxide scale as this causes more satisfactory wetting of the surface andalso makes the coating more adherent. The oxidised article may be dippedinto the solution, e. g., into a solution of any ap-.

propriate concentration, e. g. about 10%, while 1 hot and withdrawnwhile still hot enough toevaporate oil the solvent, or it may be dippedtold and then heated to remove the solvent and decompose the salt of thelong-life element to oxide. Solutions of the nitrates of the long-lifeelements may be most conveniently used in most cases and slightly acidsolutions containing about 10% by weight of the salt give a film of thedeired thickness.

When anodic deposition is used, the article to he treated should have aclean metallic surface produced by bright annealing, pickling or sandblasting, and should be degreased before placing in the depositing bathThe bath may contain any soluble salt of the long-life elements of anyconcentration from about 10% to saturation, and this should be madeprogressively more alkaline aluminium and the like.

until precipitation commences. Acid should then be added until theprecipitate disappears and then a further slight excess. tions may bevaried over a wide range, but successful deposition may be obtained byusing a 10% solution, making the article to be plated the anode andincreasing the applied voltage from about 2 volts until deposition isindicated by the evolution of gas round the article being treated. Ifthe voltage is increased further the deposition becomes more rapid, butthe gas evolution tends to strip off the film. The gelatinous filmdeposited should have a thickness of'a few thousandths of an inch. Aftercoating the article is heated to dehydrate the film and to convert it tooxide.

Sometimes I may apply the film to the alloy or partly-fabricated articleand thereafter fabricate, or complete the fabrication of, the article,and then I may use other methods of forming the film. For instance, thesurface of the alloy or partly-fabricated article may be provided withan integral film by cementation, brought about by heating the alloy in apowdered long-life element in an inert atmosphere, or in a mixture of anoxide or other compound of the element with a reducing agent. Again, thealloy or partly-fabricated article may be passed through a bath of themolten element or this may be electrolytically deposited from a bath ofa molten salt of it. Of course these methods may also be used to formfilms on finished articles, but for this purpose the two methods firstdescribed are preferred because of their simplicity.

Again, the condi-.

The following results have been obtained in tests on an 80/20nickel-chromium alloy having a life of 33 hours at 1200 C.

Treatment Hours Alloy dipped lll solution of mlschmctalP' nitrate 59Alloy coutod nnodically with mischmetall" oxide 65 Alloy dlppod insolution of thorium nitrate 54 Alloy coated anodlcally with thorln(thorium dioxide). 65 Alloy dippod lll calcium-thorium solution 60nickel-chromium, nickel-cobalt-chromium, co-'- -balt-iron-chromium,iron-chromium, iron-chromium-aluminium, and complex alloys based onsystems such as these alloys but containing in addition one or more ofthe elements molybdenum, tungsten, titanium, columbium, zirconium, Thesealloys include those containing about 10 to 35% chromium, and mayoptionally contain from a small amount, say about 0.01%. to about 20% ofone or more metals from the group consisting of molybdenum, tungsten,titanium, columbium, zirconium, aluminium, silicon and manganese, andthe balance substantially all metal of the iron group, i. e. metal ofthe group consisting of nickel, iron and cobalt. As will be apparent tothose skilled in the art, these alloys may also contain small amounts ofincidental elements and impurities, e. g. sulphur, phosphorus, etc.While the total of iron group metals will usually exceed about 50%, theinvention contemplates not only those alloys in which one of the irongroup metals exceeds about 50% but also those alloys in which no oneparticular iron group metal exceeds about 50% although the sum of allthe iron group metals will exceed about 50%.

Although the present invention has been described in conjunction withpreferred embodiments, it is understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such variations and modifications are considered to bewithin the purview and scope of the appended claims.

I claim:

1. A method of improving the high temperature service life ofchromium-containing heatresisting alloys substantially free fromlong-life elements which comprises providing such alloys with anadhering surface film of at least one member selected from the groupconsisting of thorium, calcium, zirconium and the rare earth and saltsthereof that will decompose to oxides at high temperatures.

3. Electrical resistance wires, tapes, ribbons and the like made of achromium-containing heatresistlng alloy and having a surface film of atleast one member selected from the group consisting of thorium, calcium,zirconiumandthe rare earth elements, oxides and hydroxides thereof .andsalts thereof that will decompose to oxides at high temperatures,

4. An alloy composed essentially of chromium, nickel and iron andcontaining from 10 to 30% chromium, 20 to 90% nickel and to 50% iron,said alloy being provided on its surface with a film comprising at leastone member selected from the group consisting of thorium, calcium,zirconium and the rare earth elements, oxides and hydroxides thereof andsalts thereof that will decompose to oxides at high temperatures.

5. A heat-resisting chromium-containing aly provided on its surface witha film comprising a member selected from the group consisting ofmischmetall" or compounds thereof.

6. A heat-resisting chromium-containing alloy provided on its surfacewith a film comprising a member selected from the group consisting ofthorium and compounds thereof.

7. A method of increasing the service life of an article that is madefrom a heat-resisting chromium-containing alloy and that by its natureis necessaril subjected to high temperatures in service, comprisingforming a film on the surface of said article by dipping said article ina solution of at least one memberof the group consisting of the salts ofthorium, calcium, zirconium and the rare earth metals.

8. A method of increasing the service life of an article that is madefrom a heat-resisting chromium-containing alloy and that by its natureis necessarily subjected to high temperatures in service, comprisingforming a film .on the surface of said article by anodic depositionthereon of at least one member of the group consisting of the oxides andhydroxides of thorium, calcium, zirconium and the rare earth metals.

9. A method of increasing the service life of an article that is madefrom a heat-resisting chromium-containing alloy and that by its natureis necessarily subjected to high temperatures in service, comprisingapplying to the allo a film of at least one member selected from thegroup consisting of thorium, calcium, zirconium and the rare earthelements, oxides and hydroxides thereof and salts thereof that willdecompose to oxides at high temperatures, and thereafter fabricating thearticle from said alloy,

10. A method of increasing the service life of an article that is madefrom a heat-resisting chromium-containing alloy and that by it nature isnecessarily subjected to high temperatures in service, comprising partlyfabricating the article from said alloy, applying to thepartly-fabricated alloy a film of at least one member selected from thegroup consisting of thorium, calcium, zirconium and the rare earthelements, oxides and hydroxides thereof and salts thereof that willdecompose to oxides at high temperatures, and thereafter completing thefabrication of said article.

LEONARD BESSEMER PFEIL.

